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This image is a schematic of a comet. The center part of the comet, or nucleus, is represented by the flame. The solar wind particles are shown as green dots with arrows. And the ionized particles are shown as green/red dots with arrows. Neutral particles are shown by the other dots (without arrows).
Click on image for full size JPL

The Comet Coma

As the ices of the comet nucleus evaporate, they expand rapidly into a large cloud around the central part of the comet.

This cloud, called the coma, is the atmosphere of the comet and can extend for millions of miles. The cloud is very thin, however, 10,000 times thinner than a cloud in the Earth's atmosphere!

The neutral particles that are in the coma can actually become excited by the solar wind causing the particles to become ions. A continual stream of neutral particles is produced as long as the nucleus is evaporating, and these neutral particles are continually converted to ions. These ions are what help form the comet tail.

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